Vietnamese police have detained dozens of protesters in the capital, where demonstrators gathered for the second time in a week to denounce a Taiwanese company they accuse of causing fish deaths in central coastal provinces.

A group of protesters sat on the bank of a big lake in Hanoi on Sunday before police shepherded them on to a waiting bus, witnesses told the Reuters news agency. Demonstrators were also put on buses at a square in front of the nearby Hanoi Opera House.

The fish mass deaths emerged a month ago in the central province of Ha Tinh, where the Taiwanese Formosa company runs a steel plant. Fish were also washed ashore in three other provinces along a stretch of 200km.

An initial official investigation did not blame the fish deaths on Formosa's $10.6bn coastal steel plant and Formosa denies any wrongdoing.

As the scandal unfolded in April a Formosa communications official was sacked after he said Vietnam needed "to choose whether to catch fish and shrimp or to build a state-of-the-art steel mill.

"You cannot have both," the official said.

The company later apologised for the comments and has launched its own inquiry but public anger is growing.

Rare demonstrations

"Never has the Vietnamese sea been this badly polluted," army veteran Nguyen Manh Trung, 68, told the AFP news agency.

Last week, hundreds took to the streets in Hanoi and Ho Chi Minh City, Vietnam's second-largest city, to vent their anger.

Demonstrations are rare in Vietnam and are often quickly suppressed by uniformed and plain-clothes police. State-controlled media has not reported any of the demonstrations.

Last year Vietnam earned $6.6bn from seafood exports [Reuters]

The government has invited experts from Germany, Japan, the US and Israel to inspect the Ha Tinh site in an attempt to find the cause of the fish deaths.

The inspectors have yet to announce their findings.

The government's initial investigation said the cause could be "red tide", when algae blooms and produces toxins, or a release of toxic chemicals by human beings.